Eddy Current Inspection Device, Eddy Current Inspection Probe, and Eddy Current Inspection Method

ABSTRACT

Provided is an eddy current inspection device, an eddy current inspection probe and an eddy current inspection method that make it possible to detect defects existing in deeper parts of test objects. Three or more odd number of excitation coils are arranged at even intervals in a circumferential direction on a postulated circumference. Excitation currents applied to the excitation coils are controlled so that the phase difference between excitation currents applied to adjacent ones of the excitation coils arranged in the circumferential direction on the postulated circumference equals one cycle divided by the number of excitation coils. A magnetic field generated according to an eddy current occurring in the test object due to a magnetic field caused by the application of the excitation currents to the excitation coils is detected by use of a detector arranged on a postulated plane containing the postulated circumference but inside the postulated circumference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an eddy current inspection device, aneddy current inspection probe and an eddy current inspection method fordetecting a characteristic change of an inspection target (test object)such as a flaw or a change in material quality occurring on the surfaceor inside of the inspection target.

2. Description of the Related Art

The eddy current inspection method involves detecting a characteristicchange such as a flaw or a change in material quality (hereinafterreferred to as a “defect”) occurring on the surface or inside of thetest object by measuring a change in an eddy current caused in the testobject by time-varying magnetic fluxes. The time-varying magnetic fluxesare generated by use of an excitation coil placed in the vicinity of thesurface of the test object by applying time-varying electric current(excitation current) to the excitation coil.

Conventional techniques employing the eddy current inspection methodinclude, for example, a technique disclosed in JP-05-99901-A in regardto an eddy current inspection device. The eddy current inspection deviceaccording to JP-05-99901-A comprises a plurality of excitation coils,rotational magnetic field generating means, a plurality of detectioncoils, and flaw detection means. The excitation coils are attached to amagnetic core and arranged at fixed angular intervals for the purpose ofdetecting the presence/absence of a defect and the direction of eachdefect in a flat metal plate, etc. The rotational magnetic fieldgenerating means magnetically excites the excitation coils with a signalgenerated by amplitude modulation by use of AC voltage and therebyapplies a rotational magnetic field to the test object. The detectioncoils are arranged on the magnetic core corresponding to the excitationcoils in order to detect a magnetic field generated from the test objectaccording to an eddy current occurring in the test object. The flawdetection means detects defects existing in the test object based onoutput signals from the detection coils.

SUMMARY OF THE INVENTION

However, defects in the test object can develop not only at the surfaceof the test object or in the vicinity of the surface but also in adeeper part. Therefore, defect inspection in deeper parts of testobjects is being required in order to further improve the reliability ofthe defect inspection of test objects.

The eddy current in the test object caused by the magnetic fluxes fromthe excitation coils has a tendency to develop to a deeper part of thetest object with the decrease in the frequency of the excitationcurrents. However, there exists the problem that the magnetic fieldinformation acquired from the deep part of the test object is far moredeficient compared to the magnetic field information from around thesurface of the test object even at still lower frequencies. Therefore,the defect inspection in deep parts of test objects has been extremelydifficult.

The object of the present invention, which has been made inconsideration of the above situation, is to provide an eddy currentinspection device, an eddy current inspection probe and an eddy currentinspection method that make it possible to detect defects existing indeeper parts of test objects.

(1) To achieve the above object, the present invention provides an eddycurrent inspection device comprising: a main unit which has a functionof generating and outputting a fundamental signal as the foundation forgenerating excitation currents to be used for inspection of a testobject; an eddy current inspection probe which is provided separatelyfrom the main unit to be able to scan the test object while moving alongthe surface of the test object, the eddy current inspection probeincluding three or more odd number of excitation coils which arearranged at even intervals in a circumferential direction on apostulated circumference to be excited by the excitation currentsapplied thereto and a detector which is arranged on a postulated planecontaining the postulated circumference but inside the postulatedcircumference to detect a magnetic field generated according to an eddycurrent occurring in the test object due to a magnetic field caused bythe application of the excitation currents to the excitation coils; andexcitation current generating means which generates the excitationcurrents based on the fundamental signal so that the phase differencebetween excitation currents applied to adjacent ones of the excitationcoils arranged in the circumferential direction on the postulatedcircumference in the eddy current inspection probe equals one cycledivided by the number of excitation coils.

(2) Preferably, in the above eddy current inspection device (1), theexcitation current generating means is provided integrally with the mainunit.

(3) Preferably, in the above eddy current inspection device (1), theexcitation current generating means is provided integrally with the eddycurrent inspection probe.

(4) Preferably, in the above eddy current inspection device (1), theexcitation current generating means is provided separately from the mainunit or the eddy current inspection probe.

(5) Preferably, in the above eddy current inspection device (1), thedetector of the eddy current inspection probe is arranged at the centerof the postulated circumference.

(6) Preferably, in the above eddy current inspection device (1), thedetector of the eddy current inspection probe is arranged at a positionshifted from the center of the postulated circumference.

(7) To achieve the above object, the present invention provides an eddycurrent inspection probe comprising: a detection function unit includingthree or more odd number of excitation coils which are arranged at evenintervals in a circumferential direction on a postulated circumferenceto be excited by excitation currents applied thereto and a detectorwhich is arranged on a postulated plane containing the postulatedcircumference but inside the postulated circumference to detect amagnetic field generated according to an eddy current occurring in atest object due to a magnetic field caused by the application of theexcitation currents to the excitation coils; and excitation currentgenerating means which generates the excitation currents based on afundamental signal supplied from a main unit having a function ofoutputting the fundamental signal as the foundation for generating theexcitation currents to be used for inspection of the test object, theexcitation current generating means generating the excitation currentsso that the phase difference between excitation currents applied toadjacent ones of the excitation coils arranged in the circumferentialdirection on the postulated circumference in the eddy current inspectionprobe equals one cycle divided by the number of excitation coils.

(8) To achieve the above object, the present invention provides an eddycurrent inspection probe comprising a detection function unit. Thedetection function unit includes: three or more odd number of excitationcoils which are arranged at even intervals in a circumferentialdirection on a postulated circumference to be excited by excitationcurrents applied thereto; and a detector which is arranged on apostulated plane containing the postulated circumference but inside thepostulated circumference to detect a magnetic field generated accordingto an eddy current occurring in a test object due to a magnetic fieldcaused by the application of the excitation currents to the excitationcoils. The excitation coils are supplied with excitation currentsgenerated so that the phase difference between excitation currentsapplied to adjacent ones of the excitation coils arranged in thecircumferential direction on the postulated circumference in the eddycurrent inspection probe equals one cycle divided by the number ofexcitation coils based on a fundamental signal supplied from a main unithaving a function of outputting the fundamental signal as the foundationfor generating the excitation currents to be used for inspection of thetest object.

(9) Preferably, in the above eddy current inspection device (7) or (8),the detector is arranged at the center of the postulated circumference.

(10) Preferably, in the above eddy current inspection device (7) or (8),the detector is arranged at a position shifted from the center of thepostulated circumference.

(11) To achieve the above object, the present invention provides an eddycurrent inspection method comprising the steps of: generating excitationcurrents for three or more odd number of excitation coils arranged ateven intervals in a circumferential direction on a postulatedcircumference so that the phase difference between excitation currentsapplied to adjacent ones of the excitation coils arranged in thecircumferential direction on the postulated circumference equals onecycle divided by the number of excitation coils; applying the generatedexcitation currents to the excitation coils; and detecting a magneticfield generated according to an eddy current occurring in a test objectdue to a magnetic field caused by the application of the excitationcurrents to the excitation coils by use of a detector arranged on apostulated plane containing the postulated circumference but inside thepostulated circumference.

According to the present invention, defects existing in deeper parts oftest objects can be detected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram showing the outline of the overallconfiguration of an eddy current inspection device in accordance with afirst embodiment of the present invention.

FIG. 2 is a perspective view schematically showing the arrangement ofexcitation coils and a detection coil in an eddy current inspectionprobe of the eddy current inspection device.

FIG. 3 is a plan view schematically showing the arrangement of theexcitation coils and the detection coil in the eddy current inspectionprobe of the eddy current inspection device.

FIG. 4 is a graph showing the interrelationship among excitationcurrents respectively applied to the excitation coils of the eddycurrent inspection probe.

FIG. 5 is a top view showing the directions and the intensities of amagnetic field generated by the eddy current inspection probe at time 0.

FIG. 6 is a top view showing the directions and the intensities of themagnetic field generated by the eddy current inspection probe at timeT/6.

FIG. 7 is a top view showing the directions and the intensities of themagnetic field generated by the eddy current inspection probe at timeT/3.

FIG. 8 is a top view showing the directions and the intensities of themagnetic field generated by the eddy current inspection probe at timeT/2.

FIG. 9 is a perspective view showing the directions and the intensitiesof the magnetic field generated by the eddy current inspection probe attime 0.

FIG. 10 is a perspective view showing the directions and the intensitiesof the magnetic field generated by the eddy current inspection probe attime T/6.

FIG. 11 is a perspective view showing the directions and the intensitiesof the magnetic field generated by the eddy current inspection probe attime T/3.

FIG. 12 is a perspective view showing the directions and the intensitiesof the magnetic field generated by the eddy current inspection probe attime T/2.

FIG. 13 is a top view showing the directions and the intensities of aneddy current occurring at time 0 in the inspection target placed underthe eddy current inspection probe.

FIG. 14 is a top view showing the directions and the intensities of theeddy current occurring at time T/6 in the inspection target placed underthe eddy current inspection probe.

FIG. 15 is a top view showing the directions and the intensities of theeddy current occurring at time T/3 in the inspection target placed underthe eddy current inspection probe.

FIG. 16 is a top view showing the directions and the intensities of theeddy current occurring at time T/2 in the inspection target placed underthe eddy current inspection probe.

FIG. 17 is a graph showing the relationship between the eddy currentintensity and the depth from the surface of the test object (inspectiontarget).

FIG. 18 is a perspective view schematically showing the scanning of thetest object by the eddy current inspection probe.

FIG. 19 is a vertical sectional view schematically showing the scanningof the test object by the eddy current inspection probe.

FIG. 20 is a graph showing the correspondence between the scan positionand the real number component and the imaginary number component ofdetected signal voltage.

FIG. 21 is a graph showing the real number component and the imaginarynumber component of the detected signal voltage as a Lissajou's figure.

FIG. 22 is a graph showing a calibration curve in regard to the defectdepth and the amplitude of the detected signal.

FIG. 23 is a functional block diagram schematically showing principalparts of the configuration of an eddy current inspection device inaccordance with a second embodiment of the present invention.

FIG. 24 is a functional block diagram showing the outline of the overallconfiguration of an eddy current inspection device in accordance with athird embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, a description will be given in detail ofpreferred embodiments in accordance with the present invention.

First Embodiment

FIG. 1 is a functional block diagram showing the outline of the overallconfiguration of an eddy current inspection device in accordance withthe first embodiment of the present invention. FIGS. 2 and 3 are aperspective view and a plan view schematically showing the arrangementof excitation coils and a detection coil in an eddy current inspectionprobe of the eddy current inspection device.

Referring to FIG. 1, the eddy current inspection device of thisembodiment comprises a frequency information outputting unit 1, afundamental signal generating section 7, an excitation signal generatingsection 3, a magnetic field generating section 4 and a detection section5. The frequency information outputting unit 1 generates and outputsinformation on the frequency of a fundamental signal. The fundamentalsignal generating section 7 has an oscillator 2 for generating andoutputting the fundamental signal (as the foundation for generatingexcitation currents to be used for the inspection of the test object)based on the frequency information supplied from the frequencyinformation outputting unit 1. The excitation signal generating section3 generates and outputs excitation signals based on the fundamentalsignal supplied from the oscillator 2. The magnetic field generatingsection 4 generates a magnetic field to be used for the inspectionaccording to the excitation signals supplied from the excitation signalgenerating section 3. The detection section 5 detects a magnetic fieldgenerated according to an eddy current occurring in the test object dueto the magnetic field generated by the magnetic field generating section4.

The excitation signal generating section 3 includes a plurality ofexcitation signal generating units 3A-3C, . . . for respectivelygenerating excitation signals corresponding to a plurality of (five inthis embodiment) excitation coils 61 a-61 e (see FIGS. 2 and 3) based onthe fundamental signal supplied from the oscillator 2. Among the signalgenerating units 3A-3C, . . . , only those corresponding to theexcitation coils 61 a-61 c are shown in FIG. 1 as representatives.

The excitation signal generating unit 3A includes an amplitudeadjustment unit 31 a which adjusts the amplitude of the fundamentalsignal supplied from the oscillator 2, a phase adjustment unit 32 awhich adjusts the phase of the fundamental signal and outputs theadjusted signal as the excitation signal, and an amplitude/phase controlunit 33 a which controls the operation of the amplitude adjustment unit31 a and the phase adjustment unit 32 a based on a measurement resultinputted from an excitation current measuring unit 43 a which will beexplained later.

The other excitation signal generating units 3B, 3C, . . . also haveequivalent configurations. Specifically, each excitation signalgenerating unit 3B, 3C, . . . includes an amplitude adjustment unit 31b, 31 c, . . . which adjusts the amplitude of the fundamental signalsupplied from the oscillator 2, a phase adjustment unit 32 b, 32 c, . .. which adjusts the phase of the fundamental signal and outputs theadjusted signal as the excitation signal, and an amplitude/phase controlunit 33 b, 33 c, . . . which controls the operation of the amplitudeadjustment unit 31 b, 31 c, . . . and the phase adjustment unit 32 b, 32c, . . . based on an excitation current measurement result inputted froman excitation current measuring unit 43 b, 43 c, . . . which will beexplained later.

The magnetic field generating section 4 includes a plurality of magneticfield generating units 4A-4C, . . . for respectively generating magneticfields according to the excitation signals supplied from the excitationsignal generating units 3A-3C, . . . of the excitation signal generatingsection 3. Among the magnetic field generating units 4A-4C, . . . onlythose corresponding to the excitation coils 61 a-61 c are shown in FIG.1 as representatives.

The magnetic field generating unit 4A includes the excitation coil 61 a,a power amplifier 41 a, an excitation current detecting unit 42 a, andan excitation current measuring unit 43 a. The power amplifier 41 aamplifies the electric power of the excitation signal supplied from theexcitation signal generating unit 3A and applies the amplified electricpower to the excitation coil 61 a as the excitation current. Theexcitation coil 61 a generates a magnetic field according to theexcitation current supplied thereto. The excitation current detectingunit 42 a detects the amplitude component and the phase component of theexcitation current applied by the power amplifier 41 a to the excitationcoil 61 a. The excitation current measuring unit 43 a measures theamplitude and the phase of the excitation current based on the result ofthe detection by the excitation current detecting unit 42 a and outputsthe excitation current measurement result to the amplitude/phase controlunit 33 a of the excitation signal generating unit 3A.

The other magnetic field generating units 4B, 4C, . . . also haveequivalent configurations. Specifically, each magnetic field generatingunit 4B, 4C, . . . includes an excitation coil 61 b, 61 c . . . whichgenerates a magnetic field according to the excitation current suppliedthereto, a power amplifier 41 b, 41 c, . . . which amplifies theelectric power of the excitation signal supplied from the excitationsignal generating unit 3B, 3C, . . . and applies the amplified electricpower to the excitation coil 61 b, 61 c, . . . as the excitationcurrent, an excitation current detecting unit 42 b, 42 c, . . . whichdetects the amplitude component and the phase component of theexcitation current applied by the power amplifier 41 b, 41 c, . . . tothe excitation coil 61 b, 61 c, . . . and an excitation currentmeasuring unit 43 b, 43 c, . . . which measures the amplitude and thephase of the excitation current based on the result of the detection bythe excitation current detecting unit 42 b, 42 c, . . . and outputs theexcitation current measurement result to the amplitude/phase controlunit 33 b, 33 c, . . . of the excitation signal generating unit 3B, 3C,. . . .

The detection section 5 includes a magnetic field detecting element 62,a magnetic field distribution evaluating unit 51, a display unit 52, anda storage unit 53. The magnetic field detecting element 62 (implementedby a detection coil in this embodiment) serves as a detector fordetecting the magnetic field generated according to the eddy currentoccurring in the test object due to the magnetic field caused by theapplication of the excitation currents to the excitation coils 61 a-61c, . . . . The magnetic field distribution evaluating unit 51 extracts achange in the magnetic field distribution caused by a characteristicchange such as a flaw or a change in material quality occurring on thesurface or inside of the test object (hereinafter referred to as a“defect”) based on the result of the detection by the magnetic fielddetecting element 62. The display unit 52 displays the result of theextraction by the magnetic field distribution evaluating unit 51. Thestorage unit 53 stores the result of the extraction by the magneticfield distribution evaluating unit 51.

Incidentally, while the coil having the property of increasing the levelof the detected signal (output voltage) with the increase in thesharpness of the change in the magnetic field as the target of detection(i.e., with the increase in the amount of change of the magnetic fieldper unit time) is employed as an example of the magnetic field detectingelement 62 in this embodiment, other types of sensors directlyconverting the strength of the magnetic field into the output voltagemay also be employed in place of the coil. The employable sensorsinclude a Hall sensor, an MI (Magneto-Impedance) sensor, a GMR (GiantMagneto-Resistive) sensor, an AMR (Anisotropic Magneto-Resistance)sensor, a TMR (Tunnel Magneto-Resistance) sensor, an FG (Flux Gate)sensor, a SQUID (Superconducting Quantum Interference Device) sensor,etc.

In the configuration described above, the excitation signal generatingsection 3 and the power amplifiers 41 a-41 c, . . . the excitationcurrent detecting units 42 a-42 c, . . . and the excitation currentmeasuring units 43 a-43 c, . . . of the magnetic field generatingsection 4 form an excitation current generating section which generatesthe excitation currents according to the fundamental signal. Theexcitation current generating section, the fundamental signal generatingsection 7 and the magnetic field distribution evaluating unit 51, thedisplay unit 52 and the storage unit 53 of the detection section 5 forma main unit. The excitation coils 61 a-61 c, . . . and the magneticfield detecting element 62 form an eddy current inspection probe 6 whichis provided separately from the main unit to be able to scan the testobject while moving along the surface of the test object.

Referring to FIG. 2, the eddy current inspection probe 6 according tothis embodiment includes three or more odd number of (five in thisembodiment) excitation coils 61 a-61 e and the detection coil 62. Theexcitation coils 61 a-61 e are arranged at even intervals in thecircumferential direction on a postulated circumference 64 to be excitedby the application of the excitation currents. The detection coil 62 isarranged on a postulated plane 63 containing the postulatedcircumference 64 but inside the postulated circumference 64 to serve asthe detector for detecting the magnetic field generated according to theeddy current occurring in the test object due to the magnetic fieldcaused by the application of the excitation currents to the excitationcoils 61 a-61 e.

FIG. 2 shows a case where five excitation coils 61 a-61 e are arrangedat even intervals in the circumferential direction on the postulatedcircumference 64 and the detection coil 62 is arranged at the center ofthe postulated circumference 64 on the postulated plane 63. Thus, theexcitation coils 61 a-61 e are arranged at equal distances from thecenter of the postulated circumference 64 and at even angles θ=360(degrees)÷5 (pieces)=72 (degrees) around the center. Incidentally, whenthe number of excitation coils is three, the angle between adjacentexcitation coils is θ=120 (degrees). The angle is θ=51.4 (degrees) whenthe number of excitation coils is seven, and θ=40 (degrees) when thenumber of excitation coils is nine.

FIG. 4 is a graph showing the interrelationship among the excitationcurrents applied to the excitation coils 61 a-61 e of the eddy currentinspection probe 6. The excitation signals supplied from the excitationsignal generating section 3 to the power amplifiers 41 a, 41 b, . . . ofthe magnetic field generating section 4 are also in the sameinterrelationship. In FIG. 4, the vertical axis represents the amplitudeof each excitation current (or each excitation signal) and thehorizontal axis represents the time. Explanation of the absolute valuesof the excitation currents applied to the excitation coils 61 a-61 e (orthe excitation signals) is omitted here since the graph of FIG. 4 simplyillustrates the interrelationship among the excitation currents (oramong the excitation signals).

As shown in FIG. 4, excitation currents 161 a-161 e are applied to theexcitation coils 61 a-61 e, respectively. Specifically, excitationcurrents 161 a-161 e that have been controlled so that the phasedifference between adjacent excitation currents (applied to adjacentones of the excitation coils 61 a-61 e arranged in the circumferentialdirection on the postulated circumference 64) equals one cycle dividedby the number (5 in this example) of excitation coils 61 a-61 e (T/5 inthis example) are applied to the excitation coils 61 a-61 e of the eddycurrent inspection probe 6.

For example, assuming that the cycle of the excitation current 161 aapplied to the excitation coil 61 a equals T, the excitation current 161b for the excitation coil 61 b also has the same cycle T and is appliedto the excitation coil 61 b one fifth of the cycle (T/5) later than theexcitation current 161 a. Similarly, the excitation currents 161 c, 161d and 161 e for the excitation coils 61 c, 61 d and 61 e also have thesame cycle T and are applied T/5 later than the excitation currents 161b, 161 c and 161 d, respectively. In the same sense, the excitationcurrent 161 a for the excitation coil 61 a is applied T/5 later than theexcitation current 161 e.

To sum up, when the number of excitation coils is N (arbitrary number),each excitation current for each one of the excitation coils has thesame cycle T and is applied 1/N of the cycle (T/N) later than theadjacent excitation current applied to the adjacent excitation coil onthe postulated circumference 64.

FIGS. 5-12 are schematic diagrams showing the directions and theintensities of the magnetic field generated by the eddy currentinspection probe 6, wherein FIGS. 5-8 are top views and FIGS. 9-12 areperspective views. In the figures, the direction of the magnetic fieldat each position is indicated by the direction of each arrow, and theintensity of the magnetic field at each position is indicated by thedarkness of each arrow. Each arrow represents a stronger/weaker magneticfield with the increase/decrease in the darkness (i.e., with theincrease in the blackness/whiteness).

FIGS. 5 and 9 show the status of the magnetic field in casescorresponding to the time 0 in FIG. 4. FIGS. 6 and 10 show the status ofthe magnetic field in cases corresponding to the time T/6 in FIG. 4.FIGS. 7 and 11 correspond to the time T/3 in FIG. 4. FIGS. 8 and 12correspond to the time T/2 in FIG. 4.

In FIGS. 5 and 9, for example, it is seen that a strong upward magneticfield has occurred in the vicinity of the excitation coil 61 b (in whichthe rate of change of the applied excitation current is high at the time0) and a strong downward magnetic field has occurred in the vicinity ofthe excitation coils 61 d and 61 e (in which the rate of change of theapplied excitation current is high in the opposite direction at the time0). In FIGS. 6 and 10, it is seen that a strong upward magnetic fieldhas occurred in the vicinity of the excitation coil 61 c (in which therate of change of the applied excitation current is high at the timeT/6) and a strong downward magnetic field has occurred in the vicinityof the excitation coils 61 e and 61 a (in which the rate of change ofthe applied excitation current is high in the opposite direction at thetime T/6). Similarly, in FIGS. 7 and 11, it is seen that a strong upwardmagnetic field has occurred in the vicinity of the excitation coil 61 d(in which the rate of change of the applied excitation current is highat the time T/3) and a strong downward magnetic field has occurred inthe vicinity of the excitation coils 61 a and 61 b (in which the rate ofchange of the applied excitation current is high in the oppositedirection at the time T/3). In FIGS. 8 and 12, it is seen that a strongupward magnetic field has occurred in the vicinity of the excitationcoil 61 e (in which the rate of change of the applied excitation currentis high at the time T/2) and a strong downward magnetic field hasoccurred in the vicinity of the excitation coils 61 b and 61 c (in whichthe rate of change of the applied excitation current is high in theopposite direction at the time T/2).

As above, the eddy current inspection probe 6 operates so as tosuccessively generate the magnetic fields in the order of the excitationcoils 61 a-61 e. It is also seen that the magnetic fields in the zdirection (vertical direction) are canceled out among the excitationcoils 61 a-61 e and weakened extremely at the center of the excitationcoils 61 a-61 e, that is, at the center of the postulated circumference64 where the detection coil 62 is placed.

FIGS. 13-16 are schematic diagrams showing the status of the eddycurrent occurring in the inspection target (test object) placed underthe eddy current inspection probe 6 when the magnetic fields shown inFIGS. 5-12 are generated by controlling the excitation currents as shownin FIG. 4. FIG. 13 shows the status of the eddy current in casescorresponding to the time 0 in FIG. 4. FIGS. 14, 15 and 16 show thestatus of the eddy current in cases corresponding to the time T/6, T/3and T/2 in FIG. 4, respectively. In the figures, the direction of theeddy current at each position is indicated by the direction of eacharrow, and the intensity of the eddy current at each position isindicated by the darkness of each arrow. Each arrow represents astronger/weaker eddy current with the increase/decrease in the darkness(i.e., with the increase in the blackness/whiteness).

The eddy currents occurring in the test object due to the change in themagnetic fields generated by the excitation coils 61 a-61 e of the eddycurrent inspection probe 6 are canceled out in the z direction (verticaldirection) similarly to the cancellation of the magnetic fields.

On the other hand, in the directions of the x-y plane (i.e., in thedirections along the surface of the test object), a strong eddy currentexists at each time point. With the passage of time, that is, with thechange in the magnetic fields generated by the excitation coils 61 a-61e, the flowing direction of the eddy current changes in a rotationalmanner. Specifically, in FIG. 13, an eddy current flowing from aroundthe excitation coil 61 b in the direction between the excitation coils61 d and 61 e has occurred at the time 0. In FIG. 14, an eddy currentflowing from around the excitation coil 61 c in the direction betweenthe excitation coils 61 e and 61 a has occurred at the time T/6.Similarly, in FIG. 15, an eddy current flowing from around theexcitation coil 61 d in the direction of the excitation coil 61 a hasoccurred at the time T/3. In FIG. 16, an eddy current flowing fromaround the midpoint between the excitation coils 61 d and 61 e in thedirection of the excitation coil 61 b has occurred at the time T/2.

Here, the variation in the intensity of the eddy current in the depthdirection of the test object (z direction) will be explained referringto FIG. 17.

FIG. 17 is a graph showing the relationship between the eddy currentintensity and the depth from the surface of the test object, wherein thehorizontal axis represents the depth from the test object surface andthe vertical axis represents eddy current intensity ratio determined bynormalizing the eddy current intensity so that the intensity at the testobject surface equals 1. In FIG. 17, the eddy current intensity underthe center of the postulated circumference 64 is shown and comparedamong cases where the number of excitation coils arranged on thepostulated circumference 64 is one, three and five. As shown in FIG. 17,the drop in the eddy current intensity ratio with the increase in thedepth from the test object surface is smaller in the case where thenumber of excitation coils is three compared to the case where thenumber is one. The drop in the eddy current intensity ratio with theincrease in the depth is still smaller in the case where the number ofexcitation coils is five. The difference between the case with fiveexcitation coils and the case with three excitation coils is smallerthan the difference between the case with three excitation coils and thecase with one excitation coil. Therefore, it can be presumed that usingapproximately five or seven excitation coils for the eddy currentinspection probe 6 is the most cost effective.

Next, the eddy current inspection executed by the eddy currentinspection device and the eddy current inspection probe 6 configured asabove will be explained below. This explanation will be given of a casewhere an experimental test object having defects artificially formedfrom the test object's back side (artificial defects) is used.

FIGS. 18 and 19 are diagrams schematically showing the scanning of thetest object by the eddy current inspection probe, wherein FIG. 18 is aperspective view and FIG. 19 is a vertical sectional view.

As shown in FIGS. 18 and 19, the test object 70 has artificial defects70 a-70 c formed from the back side. The artificial defects 70 a-70 chave been formed to differ from one another in the depth from thesurface of the test object. The artificial defect 70 a is the shallowestfrom the surface, the artificial defect 70 b is at an intermediatedepth, and the artificial defect 70 c is the deepest from the surface.The artificial defects 70 a, 70 b and 70 c have been formed to bearranged in this order in the scanning direction of the eddy currentinspection probe 6. Thus, the eddy current inspection probe 6 is scannedover the artificial defects to successively cross the artificial defects70 a, 70 b and 70 c in this order.

FIGS. 20 and 21 show results of calculation by the magnetic fielddistribution evaluating unit 51 in regard to the result of the detectionby the magnetic field detecting element 62 when the test object isscanned by the eddy current inspection probe. FIG. 20 is a graph showingthe correspondence between the scan position and the real numbercomponent (Vx) and the imaginary number component (Vy) of the detectedsignal voltage. FIG. 21 is a graph showing the detected signal voltageas a Lissajou's figure having the horizontal axis representing the realnumber component (Vx) and the vertical axis representing the imaginarynumber component (Vy).

As shown in FIG. 20, in the scan of the test object by the eddy currentinspection probe 6, both the real number component (Vx) and theimaginary number component (Vy) of the detected signal voltage aresubstantially 0 at positions other than the artificial defects, that is,at positions with no defect. Therefore, also in the Lissajou's figure ofFIG. 21, the detected signal voltages are drawn almost exclusively atthe central point (where both the real number component (Vx) and theimaginary number component (Vy) equal 0).

In cases where the magnetic field detecting element 62 passes over theartificial defect 70 a, 70 b or 70 c in the scan of the test object bythe eddy current inspection probe 6, both the real number component (Vx)and the imaginary number component (Vy) change corresponding to eachartificial defect 70 a, 70 b, 70 c as indicated by the changing parts 71a, 71 b and 71 c of the detected signal voltage shown in FIG. 20. Alsoin the Lissajou's figure of FIG. 21, changes 72 a, 72 b and 72 c in thedrawing appear corresponding to the artificial defects 70 a, 70 b and 70c, respectively.

In order to detect the presence/absence of a defect inside the surfaceof the actual test object and measure the depth of each defect based onthe above detection result, a calibration curve in regard to the defectdepth and the amplitude of the detected signal is generated as shown inFIG. 22 from the result of the measurement of the artificial defects 70a-70 c. The presence/absence of a defect and the depth of each defectare measured by estimation by comparing the result of detection of theactual test object with the calibration curve.

Effects achieved by this embodiment configure as above will be describedbelow.

Defects in the test object can develop not only at the surface of thetest object or in the vicinity of the surface but also in a deeper partof the test object. Therefore, defect inspection in deeper parts of testobjects is being required in order to further improve the reliability ofthe defect inspection of test objects.

The eddy current in the test object caused by the magnetic fluxes fromthe excitation coils has the tendency to develop to a deeper part of thetest object with the decrease in the frequency of the excitationcurrents. The measurable range of the characteristic change of the testobject in the depth direction is determined as a skin depth δ(m) by useof the following expression (1):

δ=(πfσμ)̂(−½)   (1)

Since the conductivity σ and the magnetic permeability μ of the testobject are fixed values in the above expression (1), it can beunderstood that the skin depth δ increases with the decrease in thefrequency f. The skin depth δ represents the depth at which the eddycurrent intensity equals 1/e of the eddy current intensity at thesurface of the test object (e: the base of natural logarithms (=2.73 . .. )).

However, there exists the problem that the magnetic field informationacquired from the deep part of the test object is far more deficientcompared to the magnetic field information from around the surface ofthe test object even at still lower frequencies. Therefore, the defectinspection in deep parts of test objects has been extremely difficult.

In contrast, in this embodiment, three or more odd number of (five inthis embodiment) excitation coils are arranged at even intervals in thecircumferential direction on a postulated circumference. The excitationcurrents applied to the excitation coils are controlled so that thephase difference between excitation currents applied to adjacent ones ofthe excitation coils arranged in the circumferential direction on thepostulated circumference equals one cycle divided by the number ofexcitation coils. The magnetic field generated according to the eddycurrent occurring in the test object due to the magnetic field caused bythe application of the excitation currents to the excitation coils isdetected by a detector arranged on a postulated plane containing thepostulated circumference but inside the postulated circumference. Withthis configuration, defects existing in deeper parts of test objects canbe detected.

Further, the eddy current inspection device is configured to detect thedefects inside the surface of the test object by use of a rotationalmagnetic field and rotational eddy current. Therefore, high detectivityindependent of the direction of each defect can be achieved.

Incidentally, while five excitation coils 61 a-61 e are arranged at evenintervals in the circumferential direction on the postulatedcircumference 64 and the detection coil 62 is arranged at the center ofthe postulated circumference 64 on the postulated plane 63 in thisembodiment, the detection coil 62 may also be arranged at a positioninside the postulated circumference 64 and shifted from the center.

Second Embodiment

A second embodiment of the present invention will be described belowwith reference to figures.

In this embodiment, the excitation signal generating section in thefirst embodiment is configured in multistage structure.

FIG. 23 is a functional block diagram showing the outline of theexcitation signal generating section, the magnetic field generatingsection and the excitation coils of the eddy current inspection devicein accordance with this embodiment. Illustration and explanation ofcomponents corresponding to the detection section in the firstembodiment are omitted for brevity. Components in FIG. 23 equivalent tothose in the first embodiment are assigned the same reference charactersas in the first embodiment and repeated explanation thereof is omittedhere.

As partially shown in FIG. 23, the eddy current inspection device ofthis embodiment comprises a frequency information outputting unit 1, afundamental signal generating section 7, excitation signal generatingunits 103A, 103B, 103C, . . . , magnetic field generating units 104A,104B, 104C, and a detection section 5. The frequency informationoutputting unit 1 generates and outputs information on the frequency ofthe fundamental signal. The fundamental signal generating section 7 hasan oscillator 2 for generating and outputting the fundamental signal (asthe foundation for generating the excitation currents to be used for theinspection of the test object) based on the frequency informationsupplied from the frequency information outputting unit 1. Theexcitation signal generating unit 103A generates and outputs anexcitation signal based on the fundamental signal supplied from theoscillator 2. The excitation signal generating unit 103B generates andoutputs an excitation signal based on the output signal from theexcitation signal generating unit 103A. The excitation signal generatingunit 103C generates and outputs an excitation signal based on the outputsignal from the excitation signal generating unit 103B. The otherexcitation signal generating units (not shown) also generate and outputexcitation signals based on the output signal from the excitation signalgenerating unit 103C. The magnetic field generating unit 104A generatesa magnetic field to be used for the inspection according to theexcitation signal supplied from the excitation signal generating unit103A. The magnetic field generating unit 104B generates a magnetic fieldto be used for the inspection according to the excitation signalsupplied from the excitation signal generating unit 103B. The magneticfield generating unit 104C generates a magnetic field to be used for theinspection according to the excitation signal supplied from theexcitation signal generating unit 103C. The other magnetic fieldgenerating units (not shown) also generate magnetic fields to be usedfor the inspection according to the excitation signals supplied from thecorresponding excitation signal generating units, respectively. Thedetection section 5 (see FIG. 1) detects a magnetic field generatedaccording to an eddy current occurring in the test object due to themagnetic fields generated by the magnetic field generating units 104A,104B, 104C, . . . .

The excitation signal generating units 103A, 103B, 103C, . . . form anexcitation signal generating section 103 which generates and outputs theexcitation signals based on the fundamental signal supplied from theoscillator 2. The magnetic field generating units 104A, 104B, 104C, . .. form a magnetic field generating section 104 which generates themagnetic fields to be used for the inspection according to theexcitation signals supplied from the excitation signal generatingsection 103. Among the excitation signal generating units 103A, 103B,103C, . . . and the magnetic field generating units 104A, 104B, 104C, .. . , only those corresponding to the excitation coils 61 a-61 c areshown in FIG. 23 as representatives.

The excitation signal generating unit 103A includes an amplitudeadjustment unit 31 a which adjusts the amplitude of the fundamentalsignal supplied from the oscillator 2, a phase adjustment unit 32 awhich adjusts the phase of the fundamental signal and outputs theadjusted signal as the excitation signal, and an amplitude/phase controlunit 33 a which controls the operation of the amplitude adjustment unit31 a and the phase adjustment unit 32 a based on a measurement resultinputted from an excitation current measuring unit 43 a.

Each excitation signal generating unit 103B, 103C, . . . includes anamplitude adjustment unit 31 b, 31 c, . . . which receives theexcitation signal from the prior excitation signal generating unit 103A,103B, . . . as a fundamental signal and adjusts the amplitude of thefundamental signal, a phase adjustment unit 32 b, 32 c, . . . whichadjusts the phase of the fundamental signal and outputs the adjustedsignal as the excitation signal, and an amplitude/phase control unit 33b, 33 c, . . . which controls the operation of the amplitude adjustmentunit 31 b, 31 c, . . . and the phase adjustment unit 32 b, 32 c, . . .based on an excitation current measurement result inputted from anexcitation current measuring unit 43 b, 43 c, . . . .

The magnetic field generating unit 104A includes the excitation coil 61a, power amplifiers 411 a and 412 a, a phase inverter 413 a, anexcitation current detecting unit 42 a, and an excitation currentmeasuring unit 43 a. The excitation coil 61 a generates a magnetic fieldaccording to an excitation current supplied thereto. The power amplifier411 a amplifies the electric power of the excitation signal suppliedfrom the excitation signal generating unit 103A and applies theamplified electric power to one end (normal phase side) of theexcitation coil 61 a as an excitation signal. The phase inverter 413 ainverts the phase of the excitation signal supplied from the excitationsignal generating unit 103A. The power amplifier 412 a amplifies theelectric power of the excitation signal supplied from the phase inverter413 a and applies the amplified electric power to the other end (reversephase side) of the excitation coil 61 a as another excitation signal.The excitation current detecting unit 42 a detects the amplitudecomponent and the phase component of the excitation current applied bythe power amplifier 411 a to the normal phase side of the excitationcoil 61 a. The excitation current measuring unit 43 a measures theamplitude and the phase of the excitation current based on the result ofthe detection by the excitation current detecting unit 42 a and outputsthe excitation current measurement result to the amplitude/phase controlunit 33 a of the excitation signal generating unit 103A.

The other magnetic field generating units 104B, 104C, . . . also haveequivalent configurations. Specifically, each magnetic field generatingunit 104B, 104C, . . . includes an excitation coil 61 b, 61 c . . .which generates a magnetic field according to the excitation currentsupplied thereto, a power amplifier 411 b, 411 c, . . . which amplifiesthe electric power of the excitation signal supplied from the excitationsignal generating unit 103B, 103C, . . . and applies the amplifiedelectric power to one end (normal phase side) of the excitation coil 61b, 61 c, . . . as an excitation signal, a phase inverter 413 b, 413 c, .. . which inverts the phase of the excitation signal supplied from theexcitation signal generating unit 103B, 103C, . . . , a power amplifier412 b, 412 c, . . . which amplifies the electric power of the excitationsignal supplied from the phase inverter 413 b, 413 c, . . . and appliesthe amplified electric power to the other end (reverse phase side) ofthe excitation coil 61 b, 61 c, . . . as another excitation signal, anexcitation current detecting unit 42 b, 42 c, . . . which detects theamplitude component and the phase component of the excitation currentapplied by the power amplifier 411 b, 411 c, . . . , to the normal phaseside of the excitation coil 61 b, 61 c, . . . , and an excitationcurrent measuring unit 43 b, 43 c, . . . , which measures the amplitudeand the phase of the excitation current based on the result of thedetection by the excitation current detecting unit 42 b, 42 c, . . . andoutputs the excitation current measurement result to the amplitude/phasecontrol unit 33 b, 33 c, . . . of the excitation signal generating unit103B, 103C, . . . .

In this embodiment, the excitation current (excitation signals) isapplied to both ends of each excitation coil 61 a, 61 b, 61 c, . . . asdifferential input by the power amplifier 411 a, 411 b, 411 c, . . . ,the power amplifier 412 a, 412 b, 412 c, . . . and the phase inverter413 a, 413 b, 413 c, . . . . Since the grounding of one end is madeunnecessary in this configuration, fluctuation in the groundingpotential occurring when the electric current value fluctuates due toimpedance fluctuation of the excitation coil 61 a, 61 b, 61 c, . . . canbe suppressed. This configuration forms an interference avoidancefunction of preventing the interference among the magnetic fieldgenerating units 104A, 104B, 104C, . . . .

The other configuration is equivalent to that in the first embodiment.

Also with this embodiment configured as above, effects similar to thoseof the first embodiment can be achieved.

Third Embodiment

A third embodiment of the present invention will be described below withreference to figures.

In this embodiment, the excitation current generating section in thefirst embodiment is provided integrally with the eddy current inspectionprobe 6.

FIG. 24 is a functional block diagram schematically showing the overallconfiguration of an eddy current inspection device in accordance withthis embodiment. Components in FIG. 24 equivalent to those in the firstembodiment are assigned the same reference characters as in the firstembodiment and repeated explanation thereof is omitted for brevity.

In FIG. 24, the eddy current inspection device of this embodimentcomprises a frequency information outputting unit 1, a fundamentalsignal generating section 7, an excitation signal generating section 3,a magnetic field generating section 4, a detection section 5A, and adetection result evaluating unit 8. The frequency information outputtingunit 1 generates and outputs information on the frequency of thefundamental signal. The fundamental signal generating section 7 has anoscillator 2 for generating and outputting the fundamental signal (asthe foundation for generating excitation currents to be used for theinspection of the test object) based on the frequency informationsupplied from the frequency information outputting unit 1. Theexcitation signal generating section 3 generates and outputs excitationsignals based on the fundamental signal supplied from the oscillator 2.The magnetic field generating section 4 generates a magnetic field to beused for the inspection according to the excitation signals suppliedfrom the excitation signal generating section 3. The detection section5A detects a magnetic field generated according to an eddy currentoccurring in the test object due to the magnetic field generated by themagnetic field generating section 4. The detection result evaluatingunit 8 evaluates and stores the result of the magnetic field detectionby the detection section 5A.

The other configuration is equivalent to that in the first embodiment.

In the configuration described above, the fundamental signal generatingsection 7 and the detection result evaluating unit 8 form a main unit.The excitation signal generating section 3 and the power amplifiers 41a-41 c, . . . , the excitation current detecting units 42 a-42 c, . . .and the excitation current measuring units 43 a-43 c, . . . of themagnetic field generating section 4 form an excitation currentgenerating section which generates the excitation currents according tothe fundamental signal. The excitation coils 61 a-61 c, . . . and themagnetic field detecting element 62 form a sensor unit 6A. The sensorunit 6A and the detection section 5A having the magnetic field detectingelement 62 form an eddy current inspection probe which is providedseparately from the main unit to be able to scan the test object whilemoving along the surface of the test object.

Also with this embodiment configured as above, effects similar to thoseof the first embodiment can be achieved.

1. An eddy current inspection device comprising: a main unit which has a function of generating and outputting a fundamental signal as the foundation for generating excitation currents to be used for inspection of a test object; an eddy current inspection probe which is provided separately from the main unit to be able to scan the test object while moving along the surface of the test object, the eddy current inspection probe including three or more odd number of excitation coils which are arranged at even intervals in a circumferential direction on a postulated circumference to be excited by the excitation currents applied thereto and a detector which is arranged on a postulated plane containing the postulated circumference but inside the postulated circumference to detect a magnetic field generated according to an eddy current occurring in the test object due to a magnetic field caused by the application of the excitation currents to the excitation coils; and excitation current generating means which generates the excitation currents based on the fundamental signal so that the phase difference between excitation currents applied to adjacent ones of the excitation coils arranged in the circumferential direction on the postulated circumference in the eddy current inspection probe equals one cycle divided by the number of excitation coils.
 2. The eddy current inspection device according to claim 1, wherein the excitation current generating means is provided integrally with the main unit.
 3. The eddy current inspection device according to claim 1, wherein the excitation current generating means is provided integrally with the eddy current inspection probe.
 4. The eddy current inspection device according to claim 1, wherein the excitation current generating means is provided separately from the main unit or the eddy current inspection probe.
 5. The eddy current inspection device according to claim 1, wherein the detector of the eddy current inspection probe is arranged at the center of the postulated circumference.
 6. The eddy current inspection device according to claim 1, wherein the detector of the eddy current inspection probe is arranged at a position shifted from the center of the postulated circumference.
 7. An eddy current inspection probe comprising: a detection function unit including three or more odd number of excitation coils which are arranged at even intervals in a circumferential direction on a postulated circumference to be excited by excitation currents applied thereto and a detector which is arranged on a postulated plane containing the postulated circumference but inside the postulated circumference to detect a magnetic field generated according to an eddy current occurring in a test object due to a magnetic field caused by the application of the excitation currents to the excitation coils; and excitation current generating means which generates the excitation currents based on a fundamental signal supplied from a main unit having a function of outputting the fundamental signal as the foundation for generating the excitation currents to be used for inspection of the test object, the excitation current generating means generating the excitation currents so that the phase difference between excitation currents applied to adjacent ones of the excitation coils arranged in the circumferential direction on the postulated circumference in the eddy current inspection probe equals one cycle divided by the number of excitation coils.
 8. An eddy current inspection probe comprising a detection function unit which includes: three or more odd number of excitation coils which are arranged at even intervals in a circumferential direction on a postulated circumference to be excited by excitation currents applied thereto; and a detector which is arranged on a postulated plane containing the postulated circumference but inside the postulated circumference to detect a magnetic field generated according to an eddy current occurring in a test object due to a magnetic field caused by the application of the excitation currents to the excitation coils, wherein: the excitation coils are supplied with excitation currents generated so that the phase difference between excitation currents applied to adjacent ones of the excitation coils arranged in the circumferential direction on the postulated circumference in the eddy current inspection probe equals one cycle divided by the number of excitation coils based on a fundamental signal supplied from a main unit having a function of outputting the fundamental signal as the foundation for generating the excitation currents to be used for inspection of the test object.
 9. The eddy current inspection probe according to claim 7, wherein the detector is arranged at the center of the postulated circumference.
 10. The eddy current inspection probe according to claim 7, wherein the detector is arranged at a position shifted from the center of the postulated circumference.
 11. An eddy current inspection method comprising the steps of: generating excitation currents for three or more odd number of excitation coils arranged at even intervals in a circumferential direction on a postulated circumference so that the phase difference between excitation currents applied to adjacent ones of the excitation coils arranged in the circumferential direction on the postulated circumference equals one cycle divided by the number of excitation coils; applying the generated excitation currents to the excitation coils; and detecting a magnetic field generated according to an eddy current occurring in a test object due to a magnetic field caused by the application of the excitation currents to the excitation coils by use of a detector arranged on a postulated plane containing the postulated circumference but inside the postulated circumference.
 12. The eddy current inspection probe according to claim 8, wherein the detector is arranged at the center of the postulated circumference.
 13. The eddy current inspection probe according to claim 8, wherein the detector is arranged at a position shifted from the center of the postulated circumference. 